This invention relates to a process for use in a drying system where combustion gases are recycled through the drying system to oxidize pollutants prior to the combustion gases being vented to the atmosphere.
Drying systems are important features in the manufacture and processing of many different materials. For example, drying systems are often used to dry wood chips during the manufacture of particle board. Further, drying systems are used during the processing of ethanol. More particularly, after ethanol has been removed from grain during a fermentation process, it is then desirable to dry the grain to allow storage and resale of the grain for animal feed or other uses.
Typical drying systems include a combustion chamber into which natural gas and air are supplied and combusted. The heated combustion gases in the combustion chamber are then induced by a draft fan into a rotating cylindrical dryer. The material to be dried is introduced into the dryer and exposed to the current of heated gases. The dried material is then separated from the heated gas current in a cyclone separator. The remaining heated gases are then vented to the environment. An example of the typical drying system of the prior art is disclosed in U.S. Pat. No. 3,861,055, which is incorporated herein by reference.
Numerous problems and disadvantages are associated with these prior art drying systems. A major problem involves the venting of the combustion gases to the atmosphere. More particularly, these combustion gases contain various pollutants. For example, the gases oftentimes contain volatile organic compounds (VOC's), carbon dioxide (CO.sub.2), and nitric oxide (NO). In addition to pollutants that result from the combustion process in the combustion chamber, pollutants can also result from the drying of the material itself. For instance, in the drying of wood chips or other organic material, particulate and VOC's are often contained in the combustion gases as they are vented to the atmosphere. Because governmental standards set the level of pollutants that can be vented to the atmosphere, it is often necessary to add additional pollution control devices to the drying systems to reduce the pollutant levels in the gas stream prior to venting. These devices often are add-on oxidizers which oxidize the VOC's and particulate present in the gas stream to reduce such pollutants to an acceptable level. These pollution control devices are typically expensive to install and operate.
Another disadvantage associated with prior art drying systems and processes involves the fire hazard associated with excessive amounts of oxygen (O.sub.2) in the combustion gases. More particularly to convey the material to be dried to the dryer, a large volume of moving gas is needed. This is especially true when the material contains a large percentage of moisture. Typically, drying systems make up the necessary volume by introducing excess air during the combustion process in the combustion chamber. Although this results in a suitable volume of gas to convey the materials, it also results in an excessive amount of O.sub.2 in the combustion gases. In many instances, the amount of O.sub.2 exceeds the allowable fire and explosion standards. The use of large amounts of excess air also results in other problems with these drying systems. More particularly, increasing the excess air admitted in the combustion chamber results in a decrease in the temperature of the combustion gases exiting the burner.
In order to reduce the amount of O.sub.2 in the combustion gases and increase the temperature levels of combustion gases to a suitable level for drying, attempts have been made to decrease the amount of excess air introduced into the combustion chamber. However, reducing the amount of excess air results in various other inherent disadvantages with the dryer system. More particularly, as is apparent, decreasing excess air results in a lower volume gas flowing through the drying chamber. This can result in ineffective and/or unstable pneumatic conveying of the product through the drying system.
Some prior art drying systems have attempted to address the above-discussed problems. More specifically, in one type of drying system, all of the combustion gases exiting the dryer are recycled back into a combustion chamber for oxidation. Gases are also taken out of the drying system at the combustion chamber and vented to the atmosphere. Recycled gases flowing into the combustion chamber and those flowing out of the combustion chamber are run through a heat exchanger wherein the heat from the gases flowing out of the combustion chamber and to the atmosphere is transferred to the recycled gases flowing into the combustion chamber. This type of drying system suffers from various disadvantages. First, because the entire quantity of combustion gases is recycled to the combustion chamber for oxidation, this drying system operates within very narrow operating parameters. More specifically, the prior art system only operates in an optimal manner at a particular capacity of the drying system. If the capacity of the drying system varies from the particular level, the oxidation temperature of the recycled gases and the inlet temperatures of the gases to the dryer could vary substantially. Because these factors could vary over large ranges, differing levels of pollutants were vented to the atmosphere depending on the capacity at which the prior art system was run. Further, again depending on the capacity, the dryer inlet temperature could vary substantially, thus resulting in inconsistent or incomplete drying of the material.
Therefore, a drying system is needed that oxidizes pollutants within the system so that external pollution control devices are not needed. Further, a drying system process is needed which decreases the amount of O.sub.2 present in the system to a level below fire standards without affecting the efficiency of the dryer due to the lack of available conveying gases. Still furthermore, a drying process is needed which will keep the oxidation temperature and dryer system efficiency all substantially constant throughout a large variance in the capacity of the drying system.